(a) Field of the Invention
The present invention relates to a transmitter/receiver having an antenna failure detection system and, more particularly, to a transmitter/receiver having an antenna failure detection system and preferably used in a base station of a mobile communication system.
(b) Description of the Related Art
An antenna failure detection system is generally used in a transmitter/receiver installed in a base station of a mobile communication system. An example of such an antenna failure detection system is described in JP-A-60-190037.
FIG. 1 shows the transmitter/receiver having an antenna failure detection system described in the publication mentioned above. The transmitter/receiver comprises a transmitting antenna 31 and a pair of receiving antennas 32 and 33, a transmitter 37 for transmitting a down-signal toward a mobile station through the transmitting antenna 31, a receiver 35 for receiving through the receiving antennas 33 and 32 an up-signal from a mobile station during a normal operation mode as well as a test signal during a test operation mode, a mobile test instrument 36 for transmitting/receiving a test signal for detection of an antenna failure, a coupler 34 for coupling transmitted signals from the mobile test instrument 36 with the transmitting/receiving antenna 31 and for coupling transmitted signals from the transmitter 37 with the transmitting antenna 31, a control unit 38 for controlling the operation of the base station, and a test unit 39 for exchanging the base station between the normal operation mode and the test operation mode.
In the test operation, the test unit 39 changes the base station from a normal operation mode to a test operation mode. The mobile test instrument 36 transmits through the antenna 31 a test signal having a format and a frequency used by a mobile station in the normal operation mode. The receiver 35 receives the test signal through the receiving antennas 32 and 33, and the control unit 38 compares the electric field of the received test signal against a reference which is equal to the electric field of the normal signals supplied from the base station without a failure in a normal operation mode, to thereby detect a failure of the base station. The test unit 39 displays an antenna failure based on the result of the comparison.
The transmitter/receiver having the mobile test instrument in addition to the transmitter and receiver, as described above, involves the problem of a large scale circuit for the base station. In addition, the number of service channels is reduced during the test operation mode because the test is conducted by using one of the service channels used in the normal communication.
JP-A-6-276166 describes another conventional antenna failure detection system in the base station, which comprises a pair of antennas 41 and 42 for receiving signals in a diversity receiving scheme, exchangers 43 and 44 disposed for antennas 41 and 42, respectively, for exchanging between transmission and reception, a receiver 46 for receiving transmitted signals through antenna 41 for modulation, a receiver 47 for receiving a transmitted signal through antenna 42 for modulation, a peak detector 48 for detecting a peak electric field of the received signal supplied from the receiver 46 in a unit time length, a peak detector 49 for detecting a peak electric field of the received signal supplied from the receiver 47 in a unit time length, a comparator 50 for comparing both the peak electric fields to output a difference signal representing the difference between the peak electric fields, a failure indicator 51 for indicating occurrence of a failure based on the difference signal, and a transmitter 45 for transmitting data for transmission.
In operation, peak electric fields of the received signals from the receivers 46 and 47 operating in a diversity receiving scheme are detected by the peak detectors 48 and 49, and compared against each other in the comparator 50 in a unit time length. If the difference between the peak electric fields exceeds a threshold value due to a failure of one of the antennas 41 and 42, the comparator 50 identifies a failed antenna having a lower peak electric field. The failure indicator 51 outputs a failed signal based on the difference signal.
The conventional transmitter-receiver of FIG. 2 has a problem in that a real time detection of antenna failure cannot be obtained if the base station has a dedicated receiving antenna and there is no mobile station in the service area of the base station during the test operation. This is because the base station uses a signal supplied from a mobile station which received the test signal in the service area of the base station, and the dedicated receiving antenna cannot be used for detection of reflected power by using the test signal.
In addition, the failure indicator 51 may judge a normal antenna as a failed antenna if the mobile station used for the test is fixed or stopped because the failure is judged based on the difference between both the electric field. In this case, one of the received electric fields may be larger at any time even if both the antennas are normal.
Further, if both the antennas failed at the same time, the failure detection cannot be obtained because the difference of the electric fields is judged based on the premise that the antenna detecting a larger electric field is normal and provides a reference value.